The present invention relates to semiconductor devices having p-n type junction and to the process for making such devices. More particularly, this invention relates to the processes for vacuum deposition of epitaxial thin-films and off-mesa bonding of ohmic contact to the films.
Thin-films of lead salt alloys have been investigated intensively recently with particular attention to their photovoltaic properties. Especial attention has been paid to their possible use as detectors of infrared radiation.
The exploration of vacuum deposition techniques is quite recent and, for the benefit of those who may not be familiar with the pioneering efforts in this art, the following brief bibliography is made of record.
Properties of PbS.sub.1-x Se.sub.x Epilayers Deposited Onto Pbs Substrates by Hot-Wall Epitaxy, by K. Duh and H. Preier, Journal of Vacuum Science Technology, Vol. 10, pp 1360, (1975).
PbSe Heteroepitaxy by the Hot-Wall Technique, by K. Duh and H. Preier, Thin Solid Films, Vol. 27, pp 247, (1975).
Double Heterojunction PbS-PbS.sub.1-x Se.sub.x -PbS Laser Diodes With CW Operation Up to 96K, by H. Preier, M. Bleicher, W. Riedl, and H. Maier, Applied Physics Letters, Vol. 28, No. 11, (June, 1976).
PbTe and Pb.sub.0.8 Sn.sub.0.2 Te Epitaxial Films on Cleaved BaF.sub.2 Substrates Prepared by a Modified Hot-Wall Technique, by T. Kasai, D. W. Bassett, and J. Hornung, Journal of Applied Physics, Vol. 47, (July, 1976).
Double-Heterostructure PbS-PbSe-PbS Lasers with CW Operation Up to 120K, by H. Preier, M. Bleicher, W. Riedl, and H. Maier, Journal of Applied Physics, Vol. 47, (December, 1976); and,
Growth of PbTe Films Under Near-Equilibrium, by A. Lopez-Otero, Journal of Applied Physics, Vol. 48, No. 1, January, 1977.
It is well established that single crystal films of PbS, PbSe, and related compounds hereinafter referred to as lead salt alloys, can be epitaxially grown on heated alkali halide substrates by vacuum evaporation. Anion (lead) vacancies make these crystals P-type and cation vacancies make them N-type.
Previous processes for creating planar junctions in thin-films of semiconducting materials subjected the films to photolithography and chemical etching. Under such processes the surface of a film is partially covered with a photo-resist pattern which acts as a mask and the film is subjected to ion implantation in order to form a region of the opposite type conductivity in the uncovered part of the film. After ion implantation the photo-resist pattern is removed by chemical action. Next, an insulating layer from two angstroms to five thousand angstroms thick of an electrically insulating material such as silicon dioxide is deposited upon the film. One or more windows in the insulating layer, each centered upon one of the conductivity regions of the film, are opened by further photolithographic and chemical etching steps. Then a film about six thousand angstrom units thick of an electrically conducting metal such as gold is vacuum deposited in a pattern running across the insulating layer from each of the windows. Electrical leads, 0.001 inch diameter wires, are bonded to the gold contacts, and the entire assembly is packaged in a multiple terminal flat pack.
The noun "semiconductor," as used here, indicates an electrically conducting medium with a resistivity in the range between those of metals and insulators, and in which the conduction is by electrons and holes. The adjective "epitaxial" means a smooth, continuous single-crystal film grown on a substrate, such that the crystal structure of the film corresponds to and is determined by that of the single-crystal substrate. More generally, epitaxial growth could refer to any situation in which crystallites grown onto a crystalline substrate in conformance with the structure of the substrate but, since for practice of the presently disclosed process continuous single-crystal layers of uniform thickness are preferable, the more restricted definition of the term epitaxy is used here. A "mesa" describes the topography of an electrical device that is supported by a substrate and has a flat top bounded abruptly by steep cliff-like sides that intersect the flat top at nearly right angles. In distinction, a "plateau" describes the level majority area of a structure that rises above the supporting substrate with uniformly sloping sides gradually leading from the surface of the substrate. "Off-mesa" bonding describes a structural feature of mesa type devices in which electrical leads are connected to the device at a point not on the mesa, typically on the substrate surface.